Electric contact and socket for electrical part

ABSTRACT

A contact pin includes a base material composed of a material having a conductive property and an outermost surface layer made of a material into which Sn is dissolved and diffused by applying heat.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of application U.S. Ser. No.11/719,465, filed May 16, 2007, the disclosure of which is herebyincorporated herein by reference. Moreover, this application is based onand claims priority to International Application No. PCT/JP2006/318870filed Sep. 22, 2006, and Japanese Application No.(s) 2005-276562 filedSep. 22, 2005, 2005-279165 filed Sep. 27, 2005, and 2006-210663 filedAug. 2, 2006, the disclosures of which are hereby incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to an electric contact to be electricallyconnected to an electrical part such as semiconductor device (called “ICpackage”, hereinafter) and a socket for an electrical part provided withsuch electric contact.

BACKGROUND ART

Conventionally, as an electric contact of the type mentioned above,there is known a contact pin provided for an IC socket as a socket foran electrical part. This IC socket is disposed on a circuit board andaccommodated with an IC package as an object to be inspected, and aterminal of this IC package and an electrode of the circuit board areelectrically connected through the contact pin.

The contact pin has an outermost Au plated layer (added with extremelysmall amount of Co) and a base layer made of Ni. On the other hand, theterminal of the IC package is made of Sn (tin) as main componentincluding no lead, so-called lead-free solder. According to the contactof the contact pin and the terminal, they are connected electrically,which are then provided for a burn-in test.

As such kind of electric contact, there is disclosed in PatentPublication 1 (Japanese Patent Publication No. 2005-504962 A)

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In such conventional one, however, there was a case in which repeatedburn-in test makes large an electric resistance in an early stage,resulting in difficulty for performing an appropriate test. Byincreasing a contacting load between the IC package terminal and thecontact pin, the increasing of the electric resistance can be suppressedby some extent, but there is a restriction on increasing the contactload and there is hence a limit on suppressing the increasing of theelectric resistance.

Then, an object of the present invention is to provide an electriccontact and a socket for an electrical part capable of suppressing theincreasing of the electric resistance and performing an appropriateburn-in test even if the test is carried out repeatedly.

Means for Solving the Problem

In order to solve the above problems, the inventors of the subjectapplication studied and found the following matters. That is, in aconventional technology, since the outermost surface layer of thecontact pin is formed of an Au plating layer and the base layer is madeof Ni, when the burn-in test with respect to an IC package having a leadfree solder terminal is repeatedly carried out, the Au is dissolved inthe terminal side, and then, the Au plating layer is vanished. Finally,Ni of the base layer is exposed. Then, the Ni is oxidized in air and anoxide film having a large specific resistance is formed. As a result, itwas found that the electric resistance at the contact portion of thecontact pin to the terminal becomes high.

From the above fact, the inventors of the subject application consideredthat, in order to prevent the outermost surface layer of the electriccontact side from being dissolved into the terminal side of theelectrical part and to prevent Ni of the base layer from being exposed,Sn in the solder of the terminal of the electrical part is transferredto the electric contact side during the burn-in (high temperature) test,and the Sn is diffused into the outermost surface layer and hardlystored as an oxide thereof. According to this matter, even if theburn-in test is repeatedly performed, the electric resistance is notincreased in an early stage.

Then, the invention of claim 1 is characterized in that the electriccontact includes a base material having conductive property and anoutermost layer composed of a material into which Sn (tin) is dissolvedand diffused by applying heat.

The invention of claim 2 is, in addition to the structure of claim 1,characterized in that the electric contact further includes a base layercomposed of Ni (nickel) formed between the base material and theoutermost surface.

The invention of claim 3 is, in addition to the structure of claim 1 or2, characterized in that the outermost surface layer is made of Pd(palladium) and Ag (silver).

The invention of claim 4 is, in addition to the structure of claim 1 or2, characterized in that the Ag has a weight larger than that of the Pd.

The invention of claim 5 is, in addition to the structure of claim 1 or2, characterized in that the outermost surface layer includes a Pd—Agplating layer.

The invention of claim 6 is, in addition to the structure of claim 1 or2, characterized in that the outermost surface layer is a lamination ofa Pd—Ag plating layer and an Ag plating layer or a Pd plating layer.

The invention of claim 7 is, in addition to the structure of claim 1 or2, characterized in that the outermost surface layer is a lamination ofan Ag plating layer and a Pd plating layer.

The invention of claim 8 is, in addition to the structure of claim 1 or2, characterized in that the outermost surface layer is a layer composedof Ag and another one or more metal.

The invention of claim 9 is, in addition to the structure of claim 8,characterized in that the outermost surface layer is composed of Ag andSn.

The invention of claim 10 is, in addition to the structure of claim 8,characterized in that the outermost surface layer includes Ag and Sn, aweight % of the Sn is not less than 80% with respect to the Sn.

The invention of claim 11 is characterized in that in an electriccontact which is electrically connected to a terminal of an electricalpart in contact therewith, a contact portion contacting to at least theterminal is formed of a composite material of Ag—an oxide of metalelement, into which Sn (tin) is dissolved and diffused by applying heat.

The invention of claim 12 is, in addition to the structure of claim 11,characterized in that the composite material is made of Ag—ZnO.

The invention of claim 13 is, in addition to the structure of claim 12,characterized in that the Ag has a weight % of not less than 80% withrespect to the ZnO.

The invention of claim 14 is, in addition to the structure of claim 11,characterized in that the composite material is made of Ag—SnO₂.

The invention of claim 15 is, in addition to the structure of claim 14,characterized in that the Ag has a weight % of not less than 80% withrespect to the SnO₂.

The invention of claim 16 is characterized in that in an electriccontact which is electrically connected to a terminal of an electricalpart in contact therewith, a contact portion contacting to at least theterminal is formed of a composite material made of Ag-a substance otherthan metal, into which Sn (tin) is dissolved and diffused by applyingheat.

The invention of claim 17 is, in addition to the structure of claim 16,characterized in that the composite material is made of Ag—C.

The invention of claim 18 is, in addition to the structure of claim 17,characterized in that the Ag has a weight % of not less than 80% withrespect to the C.

Furthermore, in the study of the inventors of the subject application,the inventors found the following matters. That is, in a conventionaltechnology, since the outermost surface layer of the contact pin isformed of an Au plating layer and the base layer is made of Ni, in therepeated burn-in test to an IC package having a lead free solderterminal, the Au is dissolved into the terminal side, and then, the Auplating layer is vanished. Finally, the Ni component of the base layeris exposed.

That is, in the burn-in test, the Au on the contact pin side and the Snof the IC package terminal forms an alloy. Then, after the test, whenthe device is taken out, almost all part of this alloy is peeled offwith the IC package terminal, and therefore, in the repetition of theburn-in test, the Au plating layer on the contact pins side is made thinand, finally, Ni of the base layer is exposed.

When Ni of the base layer is exposed, the Ni is oxidized in air andforms an oxide film having a large specific resistance. As a result, itwas found that the electric resistance of the contact portion withrespect to the terminal of the contact pin becomes high.

Then, from the above fact, the inventors of the subject applicationfound that by making the Sn diffuse into the surface layer of theelectric contact and by suppressing the peeling of the surface layer ofthe electric contact with the terminal of the electric part, an oxidefilm is hardly formed on the surface layer and the increasing of theelectric resistance can be suppressed.

That is, the invention of claim 19 is characterized in that in anelectric contact which is electrically connected to a terminal of anelectrical part in contact therewith, a contact portion contacting to atleast the terminal is provided with a contact material into which Sn(tin) included in a solder is diffused, and the contact material has atensile strength higher than that of the solder, and an alloy containingboth the contact material and the diffused Sn has a tensile strengthhigher than that of the solder.

The invention of claim 20 is, in addition to the structure of claim 19,characterized in that the contact material is formed of a plating layerof Pd(palladium)-Ag(silver) alloy, a lamination of Ag plating layer andPd—Ag alloy plating layer, a lamination of the Ag plating layer and thePd plating layer, or an Ag—Sn alloy plating layer.

The invention of claim 21 is, in addition to the structure of claim 19,characterized in that the contact material is formed by adding an oxideor organic material to a material into which Sn contained in the solderis hardly diffused therein to thereby enable Sn contained in the solderto be diffused.

The invention of claim 22 is, in addition to the structure of claim 21,characterized in that the contact material is a material prepared byadding zinc oxide, carbon or tin oxide to silver.

The invention of claim 23 is characterized in that a socket for anelectrical part includes a socket body, an accommodation portion inwhich an electrical part having a terminal including Sn is accommodated,and an electric contact recited in any one of claims 1 to 22 which isdisposed to the socket body so as to contact the terminal.

Effects of the Invention

According to the present invention of the characters mentioned above,the electric contact is composed of a base material made of a materialhaving conductivity and an outermost surface layer composed of amaterial in which Sn is diffused by applying heat, so that Sn in thesolder of the terminal of the electrical part to which the electriccontact is contacted under the burn-in test environment is transferredto the outermost surface layer and diffused therein. Accordingly, thecomponent of the outermost surface layer is not dissolved into theterminal side of the electrical part and the base layer is not exposed,and hence, the Sn is hardly stored as an oxide on the surface of thecontact portion. As a result, even in a repeated burn-in test, theincreasing of the electric resistance of the contact portion of theelectric contact in an early stage can be suppressed.

Moreover, according to another invention, the contact portion of theelectric contact is made of a composite of Ag-an oxide of metal elementor a composite of Ag-a substance other than metal so that the Sn isdiffused deeply inside the contact portion and is hardly stored as anoxide on the surface of the contact portion, thereby maintaining the lowresistance. Therefore, even if the burn-in test is repeated, theincreasing of the electric resistance of the contact portion of theelectric contact in an early stage can be effectively suppressed.

Still furthermore, according to a further invention, a contact materialin which Sn included in the solder is provided for the contact portionof the electric contact, the Sn is diffused inside the contact portion,and the Sn is hardly accumulated as an oxide on the surface of thecontact portion. In addition, this contact material has a tensilestrength higher than that of the solder, and the tensile strength of analloy containing both the contact material and the diffused Sn is alsohigher than that of the solder, so that when an electrical part is takenout, as a result, the contact portion is peeled off at a boundaryportion between the contact portion of the electric contact and thesolder. Thus, the metal on the surface of the contact portion is notpeeled off and transferred to the terminal side of the electrical part,Ni of the base layer is not exposed, and the low resistance can bemaintained, so that even if the burn-in test is repeatedly carried out,the increasing of the electric resistance of the contact portion of theelectric contact in an early stage can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a frame format, in an enlarged scale, of a sectional viewshowing a contacting portion of a contact pin according to Ag—Pd platingof a first embodiment of the present invention.

FIG. 2A to 2C are views showing a probe according to the firstembodiment.

FIG. 3A to 3C are solder material specimens according to the firstembodiment.

FIG. 4A to 4D are enlarged sectional views showing a section analyzingresult of a contact portion of a conventional probe.

FIG. 5A to 5C are enlarged views showing a surface analyzing result ofthe contact portion of the conventional probe.

FIG. 6 is a graph representing a relationship between a cycle of thecontact portion of the conventional probe and the electric resistance.

FIG. 7A to 7E are sectional views, in an enlarged scale, representing asection analyzing result of a contact portion of a probe according toAg—Pd plating of the first embodiment of the present invention.

FIG. 8 is a graph representing a relationship between a cycle of thecontact portion of the probe according to Ag—Pd plating of the firstembodiment and an electric resistance.

FIG. 9A to 9E are enlarged sectional views showing a section analyzingresult of a contact portion of a probe according to an ion plating ofthe first embodiment of the present invention.

FIG. 10 is a graph representing a relationship between a cycle of thecontact portion of the probe according to the ion plating of the firstembodiment and an electric resistance.

FIG. 11 is a frame format, in an enlarged scale, of a sectional viewshowing a contact portion of a contact pin of a modified embodimentcorresponding to FIG. 1 of the present invention.

FIG. 12 is a frame format, in an enlarged scale, of a sectional viewshowing a contact portion of a contact pin of another modifiedembodiment corresponding to FIG. 1 of the present invention.

FIG. 13 is a frame format, in an enlarged scale, of a sectional viewshowing a contact portion of a contact pin of a further modifiedembodiment corresponding to FIG. 1 of the present invention.

FIG. 14 is a frame format, in an enlarged scale, of a sectional viewshowing a contact portion of a contact pin of a further modifiedembodiment corresponding to FIG. 1 of the present invention.

FIG. 15 is a frame format, in an enlarged scale, of a sectional viewshowing a contact portion of a contact pin of a further modifiedembodiment corresponding to FIG. 1 of the present invention.

FIG. 16 is a frame format, in an enlarged scale, of a sectional viewshowing a contact portion of a contact pin of a further modifiedembodiment corresponding to FIG. 1 of the present invention.

FIG. 17 is a frame format, in an enlarged scale, of a sectional viewshowing a contact portion of a contact pin according to Ag—Sn plating ofa second embodiment of the present invention.

FIG. 18A to 18D are enlarged sectional views showing a section analyzingresult of a contact portion of a conventional probe.

FIG. 19 is a graph representing a relationship between a cycle of thecontact portion of the conventional probe and an electric resistance.

FIG. 20A to 20D are sectional views, in an enlarged scale, representinga section analyzing result of a contact portion of a probe according toAg—Sn plating of a second embodiment of the present invention.

FIG. 21 is a graph representing a relationship between a cycle of thecontact portion of the probe according to the Ag—Sn plating of thesecond embodiment and the electric resistance.

FIG. 22A to 22C are views showing a probe for evaluation test accordingto a third embodiment of the present invention.

FIGS. 23A and 23B are views showing a solder specimen for the evaluationtest according to the third embodiment.

FIG. 24A to 24D are enlarged sectional views showing a section analyzingresult of a contact portion of a conventional probe.

FIG. 25 is a graph representing a relationship between a cycle of thecontact portion of the conventional probe and an electric resistance.

FIG. 26A to 26D are sectional views, in an enlarged scale, representinga section analyzing result of a contact portion of a probe according toAg—ZnO composite of the third embodiment of the present invention.

FIG. 27 is a graph representing a relationship between a cycle of thecontact portion of the probe according to Ag—ZnO composite of the thirdembodiment and an electric resistance.

FIG. 28A to 28C are enlarged sectional views showing a section analyzingresult of a contact portion of a probe according to Ag—C composite ofthe third embodiment.

FIG. 29 is a graph representing a relationship between a cycle of thecontact portion of the probe according to Ag—C composite of the thirdembodiment and the resistance.

FIG. 30A to 30C are enlarged sectional views showing a section analyzingresult of a contact portion of a probe according to Ag—C composite ofthe third embodiment.

FIG. 31 is a graph representing a relationship between a cycle of thecontact portion of the probe according to Ag—SnO₂ composite of the thirdembodiment and the resistance.

FIG. 32A to 32D are schematic sectional views showing a contact portionof the contact pin according to a fourth embodiment of the presentinvention.

FIG. 33A, 33B, 33C, 33D, 33A′, 33B′, 33C′, 33D′ are schematic sectionalviews showing a contact portion of the contact pin according to a fourthembodiment of the present invention.

FIG. 34A to 34C are views showing a probe for evaluation test.

FIGS. 35A and 35B are views showing a solder specimen for the evaluationtest.

FIG. 36 is an enlarged sectional view showing a section analyzing resultof a contact portion of a conventional probe.

FIG. 37 is a graph representing a relationship between a cycle of thecontact portion of the conventional probe and a resistance.

FIG. 38 is a sectional view, in an enlarged scale, representing asection analyzing result of a contact portion of a probe of a contactmaterial of Pd—Ag alloy according to the fourth embodiment of thepresent invention.

FIG. 39 is a graph representing a relationship between a cycle of thecontact portion of the probe of the contact material of Pd—Ag alloyaccording to the fourth embodiment.

FIG. 40A to 40D are enlarged sectional views representing a sectionanalyzing result of a contact portion of a probe having a contactmaterial which is a material made of an Ag added with ZnO according tothe fifth embodiment of the present invention.

FIG. 41 is a graph representing a relationship between the cycle of acontact portion of the probe having the contact material which is amaterial made of an Ag added with ZnO and the resistance according tothe fifth embodiment and an electric resistance.

BEST MODE FOR EMBODYING THE INVENTION

Next, the present invention will be explained hereunder.

First Embodiment of the Invention

FIGS. 1 to 10 represent the first embodiment of the present invention.

An electric contact in this first embodiment is herein a contact pin 11arranged to an IC socket (socket for an electrical part) for a burn-intest, and electrically connects an IC package as an electrical part to acircuit board through the contact pin at the burn-in test time.

This IC package has a number of terminals on a lower surface of arectangular package body, and the terminal is made of mainly Sn andincludes no lead, so-called “lead-free solder”.

The IC socket has a socket body mounted to the circuit board and anumber of contact pins are arranged to the socket body.

The contact pin 11 is composed of, as shown in FIG. 1, a base material12, a base layer 13, and outermost surface layer 14. The base material12 is made of a material having conductivity and is herein made ofphosphor bronze, and the base layer 13 is formed from an Ni platinghaving a thickness of 2-3 μm.

The outermost surface layer 14 is made of a material into which Sn isdissolved when heated and is herein formed of a Pd—Ag plating layer ofabout 1 μm thick, and the Ag is largely set in weight ratio.

This Pd—Ag plating layer is formed by, for example, a plating method oran ion plating method.

The plating method is performed by such a manner that the Ni plating of2-3 μm thick is applied as the base layer 13, a strike gold plating iseffected thereon as a bonding layer, and the Pd plating of 0.5 μm andthe Ag plating of 0.5 μm are laminated as the outermost surface layer14. Thereafter, the thus formed lamination is heated by a thermostat toa predetermined temperature to thermally diffuse the Pd and Ag. In thistime, the weight ratio of the Pd and Ag is Pd:Ag=54:46, but this ratiomay be freely changed by regulating the film thickness of the Pd platinglayer and the Ag plating layer.

On the other hand, the ion-plating method is performed by such a mannerthat the Ni plating of 2-3 μm is applied as the base layer 13 and the Pdplating and Ag plating of 1 μm are laminated as the most outer layer 14using ion plating method. In this time, the weight ratio of Pd and Ag isPd:Ag=36:64, the Ag being larger.

The thus formed contact pin 11 is contacted to the terminal of the ICpackage to carry out the repeated burn-in tests. In a conventional test,an electric resistance of the contact pin 11 increased in an earlystage, but in the present embodiment, the increasing of the electricresistance could be suppressed and the burn-in test could beappropriately carried out.

That is, in the conventional technology, the outermost surface layer ofthe contact pin 11 is an Au plating layer and the base layer 13 is madeof Ni, so that in the repeated burn-in test of the IC package having thelead-free solder terminal, the Au is dissolved into the terminal side,and thus, the Au plating layer is vanished and Ni of the base layer 13is exposed outside. Then, the Ni is oxidized in air and an oxidationfilm having large specific resistance is formed. As a result, theelectric resistance of the contact portion of the contact pin 11 withrespect to the terminal becomes large.

However, in this embodiment, since the plating layer made of Pd and Agis provided for the outermost surface layer of the contact pin 11, Sn inthe solder of the terminal of the IC package is transferred anddispersed into the outermost surface layer 14. Accordingly, the Pd andAg are not dissolved into the IC package terminal side, and hence, Ni ofthe base layer 13 is never exposed. In addition, the Sn is hardlyaccumulated on the surface of the outermost layer 14 as oxide.

Therefore, even if the burn-in test is repeated, the increasing of theelectric resistance, in an early stage, of the contact portion of thecontact pin 11 can be suppressed.

Next, an evaluation test for confirming the effect of the presentinvention will be explained.

Herein, a probe 17 of the conventional Au plating in the burn-in testand a probe 18 having the outermost surface layer being formed of thePd—Ag plating layer according to the present invention were compared interms of their electric resistance increasing tendency.

(1) Test Content

The test was not an actual mount test using the contact pin but was amodel test using a probe and a solder specimen having simple shapes forthe reason of accurately evaluating characteristics of materials of thecontact portion. That is, it is considered that in the contacting in astate in which the contact pins are arranged to the socket for theelectrical part, much unstable factors exist, and reliability ofreproducibility of the test conditions may includes a problem.

The probe and the solder specimen used in the present model test arespecified as follows.

(2) Au Probe Specification

Phosphor bronze is used as the base material.

The probe 17 has a contact portion 17 a having an R-shape as shown inFIG. 2.

Quality control and manufacture of this contact portion 17 a was made insuch a manner that the front end portion of the base material was groundby using grinding paper having #1200 roughness, and thereafter, wasfinished by using grinding paper having #4000 roughness. Thereafter, Niplating of 2-3 μm was effected and Au plating of 0.8 μm was then appliedthereon.

(3) Pd—Ag Probe Specification

Phosphor bronze is used as the base material.

The probe 18 has a contact portion 18 a having an R-shape as shown inFIG. 2

Quality control and manufacture of the contact portion 18 a was made bya plating manufacture method and ion-plating manufacture method.

Manufacture by plating: the front end portion of the base material wasground by using grinding paper having #1200 roughness, and thereafter,was finished by using grinding paper having #4000 roughness. Thereafter,Ni plating of 2-3 μm was effected, strike Au plating was then appliedthereon as a bonding layer, Pd plating of 0.5 μm and Ag plating of 0.5μm were alternately laminated, and the Pd and Ag were thermally diffusedat a predetermined temperature by a thermostat. At this time, the weightratio of Pd to Ag was Pd:Ag=36:64.

(4) Solder Specimen Specification

Phosphor bronze was used as the base material.

A solder specimen had such a shape, as shown in FIG. 3, that a sectionin a direction normal to the longitudinal direction was approximatelysemi-circular shape.

Manufacture: Ni plating of 2-3 μm was effected to the base material as abarrier layer, and lead-free solder plating of 10 μm was then appliedthereon.

(5) Measuring Method

Resistance measuring method: four terminal method

Test Conditions

Contact load: 30 g (managed by weight)

Sliding distance: 0.5 mm (one direction)

Ambient temperature: room temperature—150° C.

Electric current: 80 mA

Test Cycle

a) Contact the probe contact portions 17 a, 18 a to the solder specimen20.

b) Slide the probe contact portions 17 a, 18 a with respect to thesolder specimen 20.

c) Measure the resistance of the probe contact portions 17 a, 18 a.

d) Heat the probes 17, 18 to a temperature of 150° C.

e) Keep the probes 17, 18 at 150° C. for 6 hours.

f) Lower the temperature to the room temperature.

g) Leave to stand for 30 minutes in a non-contact state.

h) Move the probe contact portions 17 a, 18 a to a new surface of thesolder specimen 20.

Carry out 40 cycles of the above a) to h) steps.

(6) Analysis of Section of Contact Portion

The above test cycle was carried out and the contact portions 17 a, 18 aof the respective probes 17, 18 were analyzed.

In the Au plated probe 17, when the burn-in test was repeated, the Auwas dissolved into the solder specimen 20 and vanished, and Ni of thebase layer was partially exposed.

That is, FIGS. 4A to 4D represent a result of the section analysis (8000times) after the transfer of the contact portion 17 a of the probe 17.In FIG. 4A which shows a component image, a thin color portion shows asection of the probe 17. In FIG. 4B, a thin color portion shows Au, inFIG. 4C, a thin color portion shows Sn, and in FIG. 4D, a thin colorportion shows Ni. Further, FIGS. 5A to 5C represent a result of thesurface analysis (8000 times) after the transfer of the contact portion17 a of the probe 17. In FIG. 5A which shows a component image, awhitish granular portion shows a transferred Sn and a dark granularportion Ni. In FIG. 5B, a dark color portion shows Ni, and a thin colorportion shows the transferred Sn. In FIG. 5C, a dark color portion showsthe transferred Sn and a thin color portion shows Ni. From FIGS. 5A to5C, it will be found that Ni is exposed at portion of the surface of thecontact portion 17 a which Sn is not transferred.

Then, it is considered that the exposure of the Ni to air may form anoxide film having a large specific resistance, thus increasing theelectrical contact resistance.

This will be confirmed from the graph of FIG. 6 showing a relationshipbetween the cycle number and an electric resistance. This graph wasprepared by measuring the electric resistance of the contact portion 17a at every cycle by using four samples of the probe 17, and as can beseen from this graph, it is found that as the cycle number increases,the electric resistance also increases.

On the other hand, with the probe 18 of Pd—Ag plating, Sn in the solderspecimen 20 is transferred to the probe 18 and diffused to thereby forman alloy layer with both Ag and Pd, so that the Ni is not exposed to airand the Sn is hardly stored on the contact portion 18 a as an oxide. Thecontact portion 18 a becomes Sn—Pd—Ag alloy or segregated Ag having arelatively low electric resistance.

That is, FIGS. 7A to 7E show a result of section analysis (8000 times)of the contact portion 18 a of the probe 18 after the transfer. FIG. 7Ashows a component image, and in FIG. 7A, thin color portion shows theprobe section, in FIG. 7B, thin color portion shows Ag, in FIG. 7C, thincolor portion shows Pd, in FIG. 7D, thin color portion shows Sn diffusedentirely, and in FIG. 7E, thin color portion shows Ni.

According to the above result, as shown in FIG. 7E, it is found that theNi is not exposed to air at the surface of the contact portion 18 a, andas shown in FIG. 7D, the Sn is diffused inside.

It is predicted that, in this diffusing process of the Sn, oxide filmsof various metals existing on the surface of the probe 18 of Pd—Agplating are destroyed. Further, for the Sn diffused portion, even if itis oxidized, its oxide film is physically easily destroyed by thecontact with the solder specimen 20 in comparison with an oxide film ofNi.

It is therefore considered that the increasing of the electricresistance of the contact portion 18 a can be suppressed by the abovematters.

This will be confirmed by the graph of FIG. 8 representing arelationship between the cycle number and an electric resistance. Thisgraph was prepared by measuring the electric resistance of the contactportion 18 at every cycle by using two samples of the probe 18, and ascan be seen from this graph, it is found that even if the cycle numberincreases, the electric resistance does not increase.

Further, FIGS. 9A to 9E show a result of the contact portion sectionanalysis by the Pd—Ag ion plating, and it is found that as like as thePd—Ag plating, the Ni is not exposed to air at the surface of thecontact portion and the Sn is diffused inside.

Furthermore, it is also found from the graph of FIG. 10 that even if thecycle number increases, the electric resistance does not increase.

FIGS. 11 to 16 represent modified examples different from the firstembodiment mentioned hereinabove.

The contact pin 11 shown in FIG. 11 has a Pd plating layer 14 b betweenthe Pd—Ag plating layer 14 a and the base layer 13, the contact pinshown in FIG. 12 has an Ag-plating layer 14 c between the Pd—Ag platinglayer 14 a and the base layer 13, the contact pin 11 shown in FIG. 13has a Pd plating layer 14 b on the Pd—Ag plating layer 14 a, the contactpin 11 shown in FIG. 14 has an Ag-plating layer 14 c on the Pd—Agplating layer 14 a, the contact pin 11 shown in FIG. 15 has a Pd platinglayer 14 b and an Ag-plating layer 14 c laminated in this order on thebase layer 13, and the contact pin shown in FIG. 16 has an Ag-platinglayer 14 c and an Pd plating layer 14 b laminated in this order on thebase layer 13.

In these contact pins 11, the plating layer composed of Pd and Ag isformed as the outermost surface layer of the contact pin 11, so that Snin the solder of the IC package terminal is transferred and diffusedinto the outermost surface layer 14 of the contact pin 11 under aburn-in environment (temperature of 80 to 170° C.). Accordingly, the Pdand Ag are not dissolved into the IC package terminal side, and Ni ofthe base layer 13 is not exposed. In addition, the Sn is hardly storedas oxide on the outermost surface layer 14. As a result, even if theburn-in test is repeated, the early increasing of the electricresistance of the contact portion of the contact pin 11 can beeffectively suppressed.

Second Embodiment of the Present Invention

FIGS. 17 to 21 represent the second embodiment according to the presentinvention.

The contact pin 11 according to the second embodiment is composed of, asshown in FIG. 17, a base material 12, a base layer 13 and an outermostsurface layer 14. The base material 12 is made of a conductive materialsuch as phosphor bronze herein. The base layer 13 is formed of the Niplating having a thickness of 2 to 3 μm.

Moreover, the outermost surface layer 14 is made of a material intowhich Sn is dissolved by applying heat, and Ag—Sn (Ag: 10 weight %)plating layer having a thickness of about 1 μm, herein. The weight ratioof Ag to Sn is 80 or more weight % of Ag to the remaining Weight % ofSn.

This Ag—Sn plating layer is formed by, for example, a plating method oran ion-plating method.

According to the contact pin 11 formed by such manufacturing method, theincreasing of the electric resistance can be suppressed and the burn-intest can be appropriately carried out contrary to the conventionalmethod in which the electric resistance is increased in an early stage,by repeatedly performing the burn-in test in contact of the contact pinwith the IC package terminal.

That is, in the conventional method, since the outermost surface layer14 of the contact pin 11 is made of the Au-plating layer and the baselayer 13 is made of the Ni material, in the repeated burn-in test of theIC package having the lead-free solder terminal, the Au is dissolvedinto the terminal side, the Au plating layer is vanished, and Ni in thebase layer 13 is exposed to air. The Ni is then oxidized in air and anoxide film having high specific resistance is formed. As a result, theelectric resistance of the contact portion of the contact pin 11 to theterminal is made high.

In the present embodiment, however, since the plating layer made of Agand Sn is provided for the outermost surface layer 14 of the contact pin11, Sn in the solder of the terminal of the IC package is transferredand diffused into the outermost surface layer 14 of the contact pin 11at the burn-in environment at a temperature of 80 to 170° C.Accordingly, the Sn is hardly stored as an oxide on the surface of theoutermost surface layer 14 of the contact pin 11 with the Ag and Snbeing not dissolved into the terminal side of the IC package and with Niof the base layer 13 being not exposed outside.

Therefore, even if the burn-in test is repeated, the early increasing ofthe electric resistance of the contact portion of the contact pin 11 canbe effectively suppressed.

Hereunder, an evaluation test confirming the effects of the presentinvention will be explained.

Herein, the probe 17 of the conventional Au plating and the probe 18having the outermost surface layer of the Pd—Ag plating layer in theburn-in test were compared in terms of electric resistance increasingtendency.

(1) Test Content

A model test, not a mount test using an actual contact pin, using asimple-shaped probe and solder specimen was performed. This was done toget more accurate evaluation of characteristics of a material of thecontact portion. That is, in the contact in which the contact pin ismounted to a socket for an electrical part, there are many unstablefactors, and reliability in reproducibility of the test condition mayinclude a problem.

The probe and solder specimen used for the present model test have thefollowing specifications.

(2) Au-Probe Specification.

Phosphor bronze was used as the base material.

The probe 17 has the shape such that the contact portion 17 a thereofhas an R-shape as shown in FIG. 2.

The quality control and manufacture of this contact portion 17 a weresuch that the front end of the base material was ground by a grindingpaper of #1200 roughness, and thereafter, was finished by using agrinding paper of #4000 roughness. A Ni plating was thereafter effectedso as to have a thickness of 2-3 μm, and the Au plating of 0.8 μm thickwas then applied thereon.

(3) Ag—Sn Probe Specification

Phosphor bronze was used as the base material.

The probe 18 has a shape such that the contact portion 18 a thereof hasan R-shape as shown in FIG. 2.

The quality control and manufacture of this contact portion 18 a wereeffected by the plating method and the ion-plating method.

Plating method: the front end of the base material was ground by agrinding paper of #1200 roughness, and thereafter, was finished by usinga grinding paper of #4000 roughness. A Ni plating was thereaftereffected so as to have a thickness of 2-3 μm and an Ag—Sn (Ag 10 weight%) plating of 1.0 μm was then applied thereon.

(4) Solder Specimen Specification

Glass epoxy material was used as the base material.

A solder specimen 20 had such a shape, as shown in FIGS. 3A to 3C, thata section in a direction normal to the longitudinal direction wasapproximately semi-circular shape.

Manufacture: Au was printed on the glass epoxy substrate and a lead-free(Sn-3Ag-0.5Cu) paste solder was screen-printed and then reflowed.

(5) Measuring Method

Resistance measuring method: four terminal method

Test Conditions

Contact load: 15 g (managed by weight)

Ambient temperature: room temperature—150° C.

Electric current: 80 mA

Test Cycle

a) Contact the probe contact portions 17 a and 18 a to the solderspecimen 20.

b) Measure the resistance of the probe contact portions 17 a and 18 a.

c) Heat the probes 17 and 18 to a temperature of 150° C.

d) Keep the probes 17 and 18 at 150° C. for 6 hours.

e) Lower the temperature to the room temperature.

f) Leave to stand for 30 minutes in a non-contact state.

g) Move the probe contact portions 17 a and 18 a to a new surface of thesolder specimen 20.

Carry out 40 cycles of the above a) to g) steps.

(6) Total Test Procedure

a) Observe and photograph the probes 17 a and 18 a by a microscope.

b) Repeat the test cycle mentioned above, change the solder specimen 20with new one at every 15 cycles, and at that time, observe andphotograph contact flaws of the contact portions 17 a and 18 a at theprobe front end portions and the solder specimen 20 by the microscope.

c) End the test at 40 cycles, and observe and photograph the contactflaws of the contact portions 17 a and 18 a at the probe front endportions and the solder specimen 20 by the microscope.

d) Analyze element on the surfaces of the contact portions 17 a and 18 aof the probe front end portions.

e) Analyze element of sections of the contact portions 17 a and 18 a ofthe probe front end portions.

(7) Analysis of Sectional Area of Contact Portion

The above test cycles were performed and the contact portions 17 a and18 a of the respective probes were analyzed.

For the probe 17 of the Au plating, when the burn-in test was repeated,the Au was dissolved in the solder specimen side and vanished, and apart of the base layer of Ni was exposed.

That is, FIG. 18 shows a result of the section analysis (8000 times)after the transfer of the contact portion 17 a of the probe 17, in whichFIG. 18A represents a component image having slightly thin color portionshowing the section of the probe 17, and in FIG. 18B, the thin colorportion shows Au, in FIG. 18C, the thin color portion shows Sn, and inFIG. 18D, thin color portion shows Ni. From this figure, it is foundthat Ni was exposed to air at the surface of the contact portion 17 a.

This Ni forms an oxide film in air having high specific resistance bythe exposure of the Ni, and because of this reason, it is consideredthat the contact resistance increases.

For the confirmation of this matter, FIG. 19 shows a graph representinga relationship between the cycle number and an electric resistance. Thisgraph shows the measured result of the electric resistance of thecontact portion 17 a at every cycle using four samples of the probe 17,and from this graph, it is found that as the cycle number increases, theelectric resistance also increases.

On the other hand, in the probe 18 of the Ag—Sn plating, there is nocase of vanishing the outermost surface, and hence, the base layer Ni onthe probe 18 side is never exposed. This is considered as one reason forkeeping the low resistance. In the probe 18 of the Ag—Sn plating, it ispredicted from FIG. 20 that the transfer of a substance generatedbetween the contact portion 18 a of the probe 18 side and the contactportion on the solder specimen 20 side is a diffusion of the Sn into thecontact portion on the probe 18 side from the contact portion 18 a ofthe solder specimen 20 side. In the process of this Sn diffusion, it ispredicted that the oxide layer of the respective metals of the contactportion surface is destroyed, and this is considered as another onereason for keeping the low resistance. Furthermore, it is alsoconsidered that the Sn diffused portion of the contact portion 18 a onthe probe 18 side of the Ag—Sn plating is likely destroyed physically bythe contacting to the solder specimen 20 in comparison with an oxidefilm of Ni even if the Sn diffused portion is oxidized.

That is, FIGS. 20A to 20D show the section analysis results (8000 times)after the transfer of the contact portion 18 a of the probe 18, and FIG.20A shows a component image and in FIG. 20A, thin color portion showsthe section of the probe, in FIG. 20B, thin color portion shows the Ag,in FIG. 20C, thin color portion shows the Sn, and in FIG. 20D, thincolor portion shows the Ni.

Accordingly, as shown in FIG. 20D, Ni is not exposed to air at thesurface of the contact portion 18 a, and as also shown in FIG. 20C, theSn is diffused inside.

It is considered that this shows that the increasing in the electricresistance of the contact portion 18 a is suppressed.

As confirmation of the above matter, there is a graph showing arelationship between the cycle number and the electric resistance ofFIG. 21 according to the measurement result. This graph shows themeasurement result of the electric resistance of the contact portion 18a at every cycle using two samples of the probe 18, and in comparison ofthis graph of FIG. 21 with the graph of conventional FIG. 19, it isfound that even if the cycle number increases the electric resistanceshown in FIG. 21 does not increase.

Third Embodiment of the Invention

FIGS. 22A to 31 show the third embodiment of the present invention.

An electric contact of this third embodiment is a contact pin disposedto an IC socket (socket for an electrical part) for a burn-in test, andthe IC package as electrical part and a circuit board are electricallyconnected through this contact pin at the time of the burn-in test.

The IC package has a number of terminals on a lower surface of arectangular package body, and the terminal is mainly made of Sn andincludes no lead, so-called “lead-free solder”.

The IC socket has a socket body mounted to the printed circuit board,and a number of contact pins are arranged to the socket body.

The contact pin 11 is formed from a composite material of Ag—ZnOsubstance as the composite material of an Ag-an oxide of a metalelement, into which Sn (tin) is dissolved and diffused by applying heat.In this case, the weight ratio of Ag:ZnO is 89.7:10.3, and Ag is morethan 80%.

Further, the contact pin can be made of Ag—SnO₂ substance as well as anAg—ZnO substance as a composite material of an Ag-metal oxide.

In addition, the contact pin may be made of a composite material of Ag—Csubstance as a composite material of an Ag-a substance other than metal.In this case, the weight ratio of Ag:C is 99:1, and Ag is more than 80%.

According to this matter, it is found that, by repeatedly carrying outthe burn-in test by contacting the contact pin to the terminal of the ICpackage, in the conventional technology, the electric resistance of thecontact pin was increased in an early stage, but according to thepresent embodiment, the increasing of the electric resistance can besuppressed and the burn-in test can be appropriately performed.

That is, in the conventional technology, the outermost layer of thecontact pin is formed of an Au plating layer and the base layer is madeof the Ni, so that in the repeated burn-in test of the IC package havingthe lead-free solder terminal, the Au is dissolved into the terminalside, and thus the Au plating layer is varnished and Ni of the baselayer is exposed. Then, the Ni is oxidized in air and an oxide filmhaving high specific resistance is formed. As a result, it is consideredthat the electric resistance of the contact portion of the contact pinwith respect to the terminal becomes high.

However, in this embodiment, since the contact portion of the contactpin is made of the Ag—ZnO composite or Ag—C composite, the Sn isdiffused deeply inside of the contact portion and the Sn is hardlyaccumulated as an oxide on the surface of the contact portion, thuskeeping the low resistance.

Thus, even if the burn-in test is repeated, an early increase in theelectric resistance of the contact portion of the contact pin can besuppressed.

Next, an evaluation test for confirming the advantageous effects of thepresent invention will be described.

Herein, a probe 117 of the conventional Au plating in the burn-in testand a probe 118 having the contact portion made of the Ag—ZnO compositeor a probe 119 having the contact portion made of the Ag—C compositeaccording to the present invention were compared in terms of theirelectric resistance increasing tendency by carrying out burn-in tests.

(1) Test Content

The test was not a actual mount test using the contact pin but was amodel test using probes 117, 118 and 119 having simple shapes and asolder specimen 120 for the reason of accurately evaluatingcharacteristics of materials of the contact portion. That is, it isconsidered that in the contact state in which the contact pins arearranged to the socket for the electrical part, many unstable factorsexist and reliability of reproducibility of the test conditions mayinclude a problem.

The probes 117, 118, 119 and the solder specimen 120 used in the presentmodel test are specified as follows.

(2) Au Probe Specification

Phosphor bronze is used as the base material.

The probe 117 has a contact potion 117 a having a right angle of 90degrees as shown in FIGS. 22A to 22C.

Quality control and manufacture of this contact portion 117 a was madein a manner such that the front end portion of the base material wasground by using grinding paper having #1200 roughness, and thereafter,was finished by using grinding paper having #4000 roughness. Thereafter,Ni plating of 2-3 μm was effected and Au plating of 0.8 μm was thenapplied thereon.

(3) Ag—ZnO Probe Specification

The whole is made of the Ag—ZnO composite with the weight ratio ofAg:ZnO=89.7:10.3.

The probe 118 has a contact portion 118 a having an angle of 90 degreesas shown in FIGS. 22A to 22C.

Both the surfaces of the front end portion of the contact portion 118 aare ground with a grinding paper of #1200 roughness, and thereafter,finished with a grinding paper of #4000 roughness.

(4) Ag—C Probe Specification

The whole is made of the Ag—C composite with the weight ratio ofAg:C=99:1.

The probe 119 has a contact portion 119 a having an angle of 90 degreesas shown in FIG. 22.

Both the surfaces of the front end portion of the contact portion 119 aare ground with a grinding paper of #1200 roughness, and thereafter,finished with a grinding paper of #4000 roughness.

(5) Solder Specimen Specification

Glass epoxy material was used as the base material.

A solder specimen 120 was prepared by printing Cu/NiAu to a glass epoxysubstrate, as shown in FIGS. 23A and 23B, had such a shape,screen-printing a lead-free paste solder (Sn—Ag—Cu, weight ratio of96.5:3:0.5) on the thus formed glass epoxy substrate and having the samereflowed by a surface tension to thereby form a contact portion 120 a.

(6) Measuring Method

Resistance measuring method: four terminal method

Test Conditions

Contact load: 15 g (managed by weight)

Ambient temperature: room temperature—150° C.

Electric current: 80 mA

Test Cycle

a) Contact the probe contact portions 117 a, 118 a and 119 a to thesolder specimen 120.

b) Measure the resistance of the probe contact portions 117 a, 118 a and119 a.

c) Heat the probes 117, 118 and 119 to a temperature of 150° C.

d) Keep the probes 117, 118 and 119 at 150° C. for 6 hours.

e) Lower the temperature to a room temperature.

f) Leave to stand for 30 minutes in a non-contact state.

g) Move the probe contact portions 117 a and 119 a to a new surface ofthe solder specimen 120.

Carry out 40 cycles of the above a) to g) steps.

(7) Analysis of Section of Contact Portion

The above test cycle was carried out and the contact portions 117 a, 118a and 119 a of the respective probes 117, 118 and 119 were analyzed.

In the Au plated probe 117, when the burn-in test was repeated, the Auwas dissolved into the solder specimen 120 and the Au plating layer wasvanished, and the base layer Ni was partially exposed.

That is, FIGS. 24A to 24D represent a result of the section analysis(8000 times) after the transfer of the contact portion 117 a of theprobe 117. FIG. 24A shows a component image, in which thin color portionshows a section of the probe contact portion 117 a, in FIG. 24B, thincolor portion shows Au, in FIG. 24C, thin color portion shows Sn, and inFIG. 24D, thin color portion shows Ni. Further, from FIG. 24D, it willbe found that Ni is exposed on the surface of the contact portion 117 a.

Then, it is considered that the exposure of the Ni to air may form anoxide film having a high specific resistance, thus increasing theelectrical contact resistance.

This will be confirmed from the graph of FIG. 25 showing a relationshipbetween the cycle number and the electric resistance. This graph wasprepared by measuring the electric resistance of the contact portion 117a at every cycle by using four samples of the probe 117, and as can beseen from this graph, it is found that as the cycle number increases,the electric resistance is also increased.

On the other hand, with the probe 118 of Ag—ZnO composite, Sn in thesolder specimen 120 is transferred to the probe 118 and diffused insideto thereby destroy oxide layer of the respective metals on the surfaceof the contact portion 118 a. Further, the transfer of the substancebetween the contact portion 118 a of the probe 118 side and the contactportion 120 a of the solder specimen 120 side is the transfer of the Snto the contact portion 118 a of the probe 118 side from the contactportion 120 a of the solder specimen 120 side. Accordingly, it isconsidered that the surface of the probe 118 is never vanished, and evenif Ni exists as the base layer on the probe 118 side, the Ni is neverexposed outside.

That is, FIGS. 26A to 26D show a result of section analysis (8000 times)of the contact portion 118 a of the probe 118 after the transfer. FIG.26A shows a component image, and in FIG. 26A, thin color portion showsthe section of the probe contact portion 118 a, in FIG. 26B, thin colorportion shows Ag, in FIG. 26C, thin color portion shows Zn, in FIG. 26D,thin color portion shows Sn.

According to the above result, as shown in FIG. 26D, it is found thatthe Sn is diffused inside, and it is predicted that the oxide film ofthe respective metals existing on the surface of the contact portion 118a is destroyed during the diffusion of this Sn. Further, it is alsoconsidered that in the Sn diffused portion of the contact portion 118 a,even if the surface is oxidized, the oxide film will be easilyphysically destroyed by the contact of the solder specimen 120, incomparison with the oxide film of the Ni.

In addition, it is considered that the increasing of the contactresistance of the contact portion 118 a is suppressed by the above fact.

This will be confirmed by the graph of FIG. 27 representing arelationship between the cycle number and the electric resistance. Thisgraph was prepared by measuring the electric resistance of the contactportion 118 a at every cycle by using two samples of the probe 118, andas can be seen from this graph, it is found that even if the cyclenumber increases, the electric resistance does not increase.

Furthermore, for the probe 119 of the Ag—C composite it is consideredthat the oxide layer of the respective metals on the surface of thecontact portion 119 a is destroyed by transferring the Sn in the solderspecimen 120 to the probe 119 and then diffusing it inside. In addition,the transfer of the substance between the contact portion 119 a of theprobe 119 side and the contact portion 120 a of the solder specimen 120side is the transfer of the Sn to the contact portion 119 a of the probe119 side from the contact portion 120 a of the solder specimen 120 side.Accordingly, it is considered that the surface of the probe 119 is nevervanished, and even if the Ni exists as the base layer on the probe 119side, the Ni is never exposed outside.

That is, FIGS. 28A to 28C show a result of section analysis (8000 times)of the contact portion 119 a of the probe 119 after the transfer. FIG.28A shows a component image, and in FIG. 28A, thin color portion showsthe section of the probe contact portion 119 a, in FIG. 28B, thin colorportion shows Ag, and in FIG. 28C, thin color portion shows Sn.

According to the above result, as shown in FIG. 28C, it is found thatthe Sn is diffused inside, and it is predicted that the oxide film ofthe respective metals existing on the surface of the contact portion 119a is destroyed during the diffusion of this Sn. Further, it is alsoconsidered that in the Sn diffused portion of the contact portion 119 a,even if the surface is oxidized, the oxide film will be easilyphysically destroyed by the contact to the solder specimen 120 incomparison with the oxide film of the Ni.

In addition, it is considered that the increasing of the contactresistance of the contact portion 119 a is suppressed by the above fact.

This will be confirmed by the graph of FIG. 29 representing arelationship between the cycle number and the electric resistance. Thisgraph was prepared by measuring the electric resistance of the contactportion 119 a at every cycle by using two samples of the probe 119, andas can be seen form this graph, it is found that even if the cyclenumber increases, the electric resistance does not increase rapidly.

Next, the electric resistance increasing tendencies in the burn-in testwere compared between the conventional probe 117 of Au plating and theprobe 121 of the contact portion of Ag—SnO₂ composite.

The test contents of this embodiment are similar to those of the Ag—ZnOcase and the Ag—C case mentioned hereinbefore. For example, the probe121 of the Ag—SnO₂ composite has the weight ratio of Ag:SnO₂=90.0:10.0,and both the surfaces of the front end of the probe 121 were ground by agrinding paper of #1200 roughness, and thereafter, finished with agrinding paper of #4000.

In this probe 117 of the Au plating, it is considered, as mentionedbefore, that since Ni is exposed to air and forms an oxide film havinghigh specific resistance, so that the contact resistance increases.

On the other hand, for the probe 121 of the Ag—SnO₂ composite, it wasobserved that the Sn was diffused deeply inside of the probe 121.Accordingly, in the probe 121 of the Ag—SnO₂ composite, it is consideredthat during the diffusing process of the Sn at the contact portion 121a, the destroy of the oxide layer of the respective metals at thesurface of the probe side contact portion is a reason for maintaining alow resistance. Furthermore, it is also considered that the Sn diffusedportion of the probe 121 of the Ag—SnO₂ composite will be easilyphysically destroyed, even if its surface is oxidized, by the contact tothe solder specimen 120 in comparison with the oxide film of the Ni.

FIGS. 30A to 30C show a result of section analysis (8000 times) of thecontact portion 121 a of the probe 121 after the transfer. FIG. 30Ashows a component image, and in FIG. 30A, thin color portion shows thesection of the probe contact portion 121 a, in FIG. 30B, thin colorportion shows Ag, and in FIG. 30C, thin color portion shows Sn.

According to the above result, as shown in FIG. 30C, it is found thatthe Sn is diffused inside, and it is predicted that the oxide film ofthe respective metals existing on the surface of the contact portion 121a is destroyed during the diffusion of this Sn. Further, it is alsoconsidered that in the Sn diffused portion of the contact portion 121 a,even if the surface is oxidized, the oxide film will be easilyphysically destroyed by the contact to the solder specimen 120 incomparison with the oxide film of the Ni.

In addition, it is considered that the increasing of the contactresistance of the contact portion 121 a is suppressed by the above fact.

This will be confirmed by the graph of FIG. 31 representing arelationship between the cycle number and the electric resistance. Thisgraph was prepared by measuring the electric resistance of the contactportion 121 a at every cycle by using two samples of the probe 121, andas can be seen from this graph, it is found that even if the cyclenumber increases, the electric resistance does not increase.

Fourth Embodiment of the Invention

FIGS. 32A to 32D represent the fourth embodiment of the presentinvention.

An electric contact of this fourth embodiment is a contact pin to bedisposed to an IC socket (socket for an electrical part) for a burn-intest, and the IC package as electrical part and a circuit board areelectrically connected through this contact pin at the time of theburn-in test.

This IC package has a number of terminals on a lower surface of arectangular package body, and the terminal is mainly made of Sn andincludes no lead, so-called “lead-free solder”.

The IC socket has a socket body mounted on the circuit board and anumber of contact pins are arranged to the socket body.

The contact pin has a contact portion 210 contacting the terminal 215,and as shown in FIGS. 32A to 32D, the contact portion 210 to becontacted to a terminal 215 is composed of a base material 211 of copper(Cu), a Ni (nickel) plating layer 212 formed on the surface side of thebase material 211, and a contact material 213 formed on the surface sideof the Ni plating layer 212.

This contact material 213 is a material into which Sn contained in asolder is diffused, and the contact material 213 is selected from amaterial having a tensile strength higher than that of the lead-freesolder, and being able to give a Sn-diffused alloy having a tensilestrength higher than that of the lead-free solder.

As such contact material 213, there is used a material formed from aplating layer of Pd (palladium)-Ag (silver) alloy, a laminated layer ofan Ag plating layer and a Pd—Ag alloy layer, a laminated layer of an Agplating layer and a Pd plating layer, or a plating layer of Ag—Sn alloy.

The tensile strength of the Pd—Ag alloy as the contact material 213 isabout 500 MPa. On the other hand, the strength of the solder is about 50MPa, and a tensile strength of the Pd—Ag alloy in the case where the Snis diffused in the alloy as the contact material 213 is higher than thetensile strength of the solder. Of course, the tensile strength of thelaminated layer of Ag plating layer and Pd—Ag alloy layer, the laminatedlayer of Ag plating layer and Pd plating layer, or the plating layer ofAg—Sn alloy, and the tensile strength of an alloy of these materials andthe Sn are both higher than the tensile strength of the solder.

Accordingly, although in the conventional technology, the electricresistance increased in an early stage, in this embodiment, theincreasing of the electric resistance can be suppressed and the burn-intest can be appropriately performed by contacting such contact pin tothe terminal 215 of the IC package and repeatedly carrying out theburn-in test.

That is, in the conventional technology, since the outermost surfacelayer of the contact pin is an Au plating layer and the base layer ismade of Ni, Au is dissolved into the terminal side, the Au plating layeris vanished and Ni of the base layer is exposed during the repeatedburn-in test of the IC package having the lead-free solder terminal.Then, it is considered that the Ni forms an oxide film in air havinglarge specific resistance, so that the electric resistance of thecontact portion of the contact pin to the terminal is increased.

However, according to this embodiment, the contact portion 210 of thecontact pin is made of the contact material 213 into which Sn containedin the solder is diffused, and accordingly, the Sn diffuses deeplyinside the contact portion 210, and the Sn is hardly stored as an oxideon the surface of the contact portion 210.

Furthermore, since the tensile strength of the contact portion 210 ofthe contact pin is higher than that of the solder, at the time of takingout an electrical part, the contact material 213 is peeled off at aboundary surface between the contact portion 210 and the terminal of theelectrical part. Therefore, a metal on the surface of the contactportion 210 on the electrical part terminal 215 side is not peeled off,and hence, the Ni plating layer 212 is not exposed and the lowresistance can be maintained, so that even if the burn-in test isrepeatedly carried out, the increasing of the electric resistance of thecontact portion 210 in an early stage can be suppressed.

Fifth Embodiment of the Invention

FIGS. 33A to 33D′ represent the fifth embodiment of the presentinvention.

A contact material 214 of this fifth embodiment differs from the contactmaterial 213 of the fourth embodiment.

The contact material 214 of this fifth embodiment enables Sn containedin the solder to be diffused by adding an oxide or organic material to amaterial in which Sn contained in the solder is hardly diffused.Further, this contact material 214 has a tensile strength higher thanthat of the solder, and the tensile strength of an alloy in which Sn isdiffused is also higher than that of the solder.

As this contact material 214, there is used a material made by addingzinc oxide, carbon or tin oxide to silver, for example.

A tensile strength of the material made by adding zinc oxide to silveras the contact material 214 is 150-300 MPa. On the other hand, thestrength of the solder is about 50 MPa, and a tensile strength of analloy, in the case where the Sn is diffused into the contact material214, is higher than the tensile strength of the solder. Of course, thetensile strength of the other material in which C is added to Ag or thatof a material in which SnO₂ is added to Ag, and that of an alloy ofthese materials and the Sn are higher than the tensile strength of thesolder.

In this embodiment, by adding an oxide or organic material to a materialinto which Sn contained in the solder is hardly diffused, the contactportion 210 of the contact pin is made of the contact material 214 inwhich Sn contained in the solder is diffused. Therefore, the Sn isdiffused deeply inside the contact portion 210 and the Sn is hardlystored as an oxide on the surface of the contact portion 210.

Furthermore, since the tensile strength of the contact portion 210 ofthe contact pin is higher than that of the solder, at the time of takingout an electrical part, the contact material 214 is peeled off at aboundary surface between the contact portion 210 and the terminal 215 ofthe electrical part. Therefore, a metal on the surface of the contactportion 210 on the electrical part terminal 215 side is not peeled off,and hence, the Ni plating layer 212 is not exposed and the lowresistance can be maintained, so that even if the burn-in test isrepeatedly carried out, the increasing of the electric resistance of thecontact portion 210 in an early stage can be suppressed.

Further, herein, there is no problem in function even if the Ni platinglayer 212 does not exist. In addition, even if the Ni plating layer 212and the Cu base material 212 do not exist and the contact portion 210 ismade entirely of the contact material 214, there is also no problem infunction (see FIGS. 33A′, 33B′, 33C′ and 33D′).

Hereunder, an evaluation test for confirming the effects of the presentinvention will be explained.

Herein, the probe 217 of the conventional Au plating and the probe 218provided with the contact material 213 (Pd—Ag) of the contact portion ofthe present invention or the probe 219 provided with the contactmaterial 214 (ZnO is added to Ag) were compared in terms of the electricresistance increasing tendency by carrying out the burn-in test.

(1) Test Content

A model test, not a mount test using an actual contact pin, usingsimple-shaped probes 217, 218 and 219 and solder specimen 220, wasperformed. This was done to get more accurate evaluation ofcharacteristics of a material of the contact pin. That is, this isbecause that the contact in which the contact pin is mounted to a socketfor an electrical part provides many unstable factors, and reliabilityin reproducibility of the test condition may include a problem.

The probes 217, 218 and 219 and solder specimen 220 used for the presentmodel test have the following specifications.

(2) Au-Probe Specification (Conventional)

Phosphor bronze was used as the base material.

The probe 217 has the shape such that the contact portion 217 a thereofhas an angle of 90 degrees as shown in FIGS. 34A to 34C.

The quality control and manufacture of this contact portion 217 a weresuch that the front end of the base material was ground by a grindingpaper of #1200 roughness, and thereafter, was finished by using agrinding paper of #4000 roughness. A Ni plating was thereafter effectedso as to have a thickness of 2-3 μm and the Au plating of 0.8 μm wasthen applied thereon.

(3) Pd—Ag Probe Specification

Phosphor bronze was used as the base material.

The probe 218 has the shape such that the contact portion 218 a thereofhas an angle of 90 degrees as shown in FIGS. 34A to 34C.

The quality control and manufacture of this contact portion 218 a weresuch that the front end of the base material was ground by a grindingpaper of #1200 roughness, and thereafter, was finished by using agrinding paper of #4000 roughness. A Ni plating was thereafter effectedso as to have a thickness of 2-3 μm and the Pd—Ag plating of 2-3 μmthick was then applied thereon. The weight ratio of Pd to Ag was 6:4.

(4) Ag—ZnO Probe Specification

The Ag—ZnO was used entirely.

The probe 219 has the shape such that the contact portion 219 a thereofhas an angle of 90 degrees as shown in FIGS. 34A to 34C.

The quality control and manufacture of this contact portion 219 a weresuch that the front end of the base material was ground by a grindingpaper of #1200 roughness, and thereafter, was finished by using agrinding paper of #4000 roughness. The weight ratio of the Ag:ZnO was9:1.

(5) Solder Specimen Specification

Glass epoxy material was used as the base material.

The solder specimen 220 was formed, as shown in FIGS. 35A and 35B, byprinting Cu/NiAu to the glass epoxy substrate, screen-printing alead-free solder paste (Sn—Ag—Cu (weight ratio: 96.5:3:0.5)) on thesubstrate and then having the same reflowed by the surface tension tothereby form the contact portion 120 a.

(6) Measuring Method

Resistance measuring method: four terminal method

Test Conditions

Contact load: 15 g (managed by weight)

Ambient temperature: room temperature—150° C.

Electric current: 80 mA

Test Cycle

a) Contact the probe contact portions 217 a, 218 a and 219 a to thesolder specimen 220.

b) Measure the resistance of the probe contact portions 217 a, 218 a and219 a.

c) Heat the probes 217, 218 and 219 to a temperature of 150° C.

d) Keep the probes 217, 218 and 219 at 150° C. for 6 hours.

e) Lower the temperature to the room temperature.

f) Leave to stand for 30 minutes in a non-contact state.

g) Move the probe contact portions 217 a, 218 a and 219 a to a newsurface of the solder specimen 220.

Carry out 40 cycles of the above a) to g) steps.

(7) Analysis of Sectional Area of Contact Potion

The above test cycles were carried out and the contact portions 217 a,218 a and 219 a of the respective probes 217, 218 and 219 were analyzed.

For the probe 217 of the Au plating, when the burn-in test was repeated,the Au was dissolved in the solder specimen 220 side, the Au platinglayer was vanished, and a part of the Ni base layer was exposed.

That is, FIG. 36 shows a result of the section analysis (8000 times)after the transfer of the contact portion 217 a of the probe 217, andfrom this FIG. 36, it is found that the Ni is exposed to air at thesurface of the contact portion 217 a.

This Ni forms an oxide film in air having large specific resistanceafter the exposure of the Ni, and because of this reason, it isconsidered that the contact resistance increases.

For the confirmation of this matter, FIG. 37 shows a graph representinga relationship between the cycle number and the electric resistance.This graph shows the measured result of the electric resistance of thecontact portion 217 a at every cycle using four samples of the probe217, and from this graph, it will be found that as the cycle numberincreases, the electric resistance also increases.

On the other hand, in the probe 218 of the Pd—Ag plating according thefifth embodiment, it is considered that Sn in the solder specimen 220 istransferred to the probe 218 and diffused inside thereof, so that theoxide layer of the various metals on the surface of the contact portion218 a is destroyed. In addition, the transfer of the substance causedbetween the contact portion 218 a of the probe 218 and the contactportion 220 a on the solder specimen 220 side is a transfer of the Snfrom the contact portion 220 a of the solder specimen 220 side to thecontact portion 218 a on the probe 218 side, so that the surface portionof the probe 218 is never vanished and even if the Ni exists as the baselayer, the Ni is never exposed.

That is, FIG. 38 shows a result of section analysis (8000 times) of thecontact portion 218 a of the probe 218 after the transfer. From thisFIG. 38, it is found that the Sn is diffused inside.

Further, the strength of the contact material 213 of the Pd—Ag is higherthan that of the solder, so that the metal on the surface of the contactportion 218 a is not transferred to the solder specimen 220 side, andthe Ni as the base layer is not hence exposed.

According to this fact, it is considered that the increasing of thecontact resistance of the contact portion 218 a is suppressed.

This is confirmed by the graph of FIG. 39 representing a relationshipbetween the cycle numbers and the electric resistance. This graph wasprepared by measuring the electric resistance of the contact portion 218a at every cycle by using two samples of the probe 218, and as can beseen from this graph, it is found that even if the cycle numberincreases, the electric resistance does not increase.

Furthermore, for the probe 219 of the material in which ZnO is added toAg according to the fifth embodiment, it is considered that Sn of thesolder specimen 220 is transferred to the probe 218 and diffused insidethereof, so that the oxide layer of the various metals on the surface ofthe contact portion 219 a is destroyed. In addition, the transfer of thesubstance caused between the contact portion 219 a on the probe 219 sideand the contact portion 220 a on the solder specimen 220 side is atransfer of Sn from the contact portion 220 a of the solder specimen 220side to the contact portion 219 a on the probe 219 side, so that thesurface portion of the probe 219 is never vanished and even if Ni existsas the base layer, the Ni is never exposed.

That is, FIGS. 40A to 40D show a result of section analysis (8000 times)of the contact portion 219 a of the probe 219 after the transfer. FIG.40A shows a component image, in FIG. 40B, thin color portion shows theAg, in FIG. 40C, thin color portion shows ZnO, and in FIG. 40D, thincolor portion shows Sn.

Accordingly, as shown in FIG. 40D, the Sn is diffused inside the contactportion 219 a.

Further, in the case of a probe where Ag is only used as a material ofthe probe, the Sn is not diffused in the inside of the Ag, any alloy isnot formed, and the Sn stored on the surface of the Ag is oxidized,thereby increasing the electric resistance.

From this fact, it is considered that the increasing in the electricresistance of the contact portion 219 a is suppressed.

As confirmation of the above matter, there is a graph of FIG. 41 showinga relationship between the cycle number and the electric resistanceaccording to the measurement result. This graph shows the measurementresult of the electric resistance of the contact portion 219 a at everycycle using two samples of the probe 219, and in view of this graph, itis found that even if the cycle number increases, the electricresistance does not increase.

Further, in the above embodiments, there were described examples inwhich the contact pin as “electric contact portion” is applied to the ICsocket, but the present invention is not limited to such examples and isapplicable to other examples.

EXPLANATION OF REFERENCE NUMERAL

-   -   11—contact pin    -   12—base material    -   13—base layer    -   14—outermost surface layer    -   14 a—Pd—Ag plating layer    -   14 b—Pd plating layer    -   14 c—Ag plating layer    -   118—probe of Ag—ZnO composite    -   119—probe of C composite    -   120—solder specimen    -   121—probe of Ag—SnO₂ composite    -   210—contact portion    -   211—base material    -   212—Ni plating layer    -   213, 214—contact material    -   218—probe having contact material of Pd—Ag    -   219—probe having a contact material of Ag to which ZnO is added    -   220—solder specimen

1. A socket for an electrical part, that is used for burn-in test,comprising: a socket body, comprising an accommodation portion in whichan electric terminal of the electrical part is accommodated, theelectric terminal comprising a film on a surface thereof, the filmcomprising Sn, the accommodation portion comprising an electric contactwhich contacts the electric terminal, the electric contact comprising:an intermediate layer comprising Ni, and an outermost surface layerwhich is formed directly on the intermediate layer, the outermostsurface layer comprising Ag and Sn.
 2. The socket for an electrical partaccording to claim 1, wherein a weight ratio of the Ag to the Sn is 80or more weight percent of the Ag to the remaining weight percent of theSn.
 3. The socket for an electrical part according to claim 1, whereinthe outermost surface layer of the electric contact is a Ag—Sn platinglayer.
 4. The socket for an electrical part according to claim 1,wherein Sn included in a solder is diffused into the outermost surfacelayer, and the outermost surface layer has a tensile strength higherthan a tensile strength of the solder, and an alloy containing both theoutermost surface layer and the diffused Sn has a tensile strengthhigher than the tensile strength of the solder.
 5. The socket for anelectrical part according to claim 4, wherein the outermost surfacelayer is made of an Ag—Sn alloy plating layer.
 6. The socket for anelectrical part according to claim 1, wherein the electric contactcomprises a pin.